Abstract

In this thesis the self-assembly of -conjugated (macro)molecular architectures, either through chemisorption or via physisorption, into highly ordered supramolecular nanoscopic and microscopic structures has been studied. On solid substrates structure and dynamics has been investigated on the molecular scale making use primarily of Scanning Probe Microscopies, in particular Scanning Tunneling Microscopy and Scanning Force Microscopy. This allowed to characterize a variety of phenomena occurring both at the solid-liquid interface, such as the dynamics of the single molecular nanorods (known as Ostwald ripening), the fractionation of a solution of rigid-rod polymers upon physisorption on graphite; and in dry films, i.e. the self-assembly of rigid-rod polymers into nanoribbons with molecular cross sections which can be epitaxially oriented at surfaces and the formation ordered layered architectures of disc-like molecules. In addition the electronic properties of the investigated moieties have been studied by means of Photoelectron Spectroscopies. The nanostructures that have been developed are not only of interest for nanoconstructions on solid surfaces, but also exhibit properties that render them candidates for applications in the field of molecular electronics, in particular for building molecular nanowire devices.

STM constant current image of undecanthiol on Ni supported-TSG surface. Ut = 800 mV; I = 40 pA. a) Unfiltered image with the 2D-Fourier Transform showing the periodicity of the hexagonal lattice (-phase); b) band pass filtered image. Although a high frequency noise blurs the raw image (as evident also from the FFT), the lattice of the adlayer can be seen in both images.

STM image of sample C11A3 on Ni supported-TSG surface. Ut = 500 mV; I = 50 pA. a) Unfiltered image; b) band pass filtered image. Although a high frequency noise blurs the raw image, the lattice of the adlayer can be recognized in both images.

STM current image of 2 in 1-phenyloctane imaged at the solid-liquid interface on HOPG (Ut=1.2 V , average It= 1.0 nA). 2-D crystal structure with its unit cell, averaged over several images amounts to a=(1.78±0.09) nm, b=(2.90±0.18) nm, =(113±5)°. The distance between neighboring backbones is in this case L=(1.46±0.11) nn.

STM constant current image of 1 in 1-phenyloctane imaged at the solid-liquid interface on HOPG. Ut = 1.2 V , It = 1.0 nA. Polycrystalline structure made of single crystallites with different molecular orientations. The arrow indicates a defect (two missing molecules) in a crystal lattice.

STM constant current images. Evolution of the 2-D polycrystal structure of 1 at the solid-liquid interface during 22 minutes: Ostwald ripening phenomena brought about by the reorientation of single molecules in island I, II, III, IV. The white scale bar is 20 nm. Arrow in a) indicates a domain boundary of the type shown in Fig. 6.27a.

STM constant current image of a PPE 4 at the interface between the basal plane of graphite and an organic solution in 1-phenyloctane. Ut=1.2 V , average It= 1.0 nA. This PPE, according to 1H-NMR analysis on the end-groups, possesses an average contour length 7.9 nm The molecular backbones exhibit a nematic orientation along preferred directions, as evidenced by the hexagonal pattern in the 2D-Fourier Transform. The distance between the backbones averaged over several images amounts to L=(1.62±0.10) nm.

STM current images of a PPE 4 at the interface between the basal plane of graphite and an organic solution in 1-phenyloctane. This PPE, according to 1H-NMR analysis on the end-groups, possesses an average contour length of 11.2 nm. The molecular backbones exhibit a nematic orientation along preferred directions, as evidenced by the hexagonal pattern in the 2D-Fourier Transform. a) Ut=1.7 V , average It= 0.2 nA; b) 1.35 V, average It=0.7 nA.

STM current images of a PPE 4 at the interface between the basal plane of graphite and an organic solution in 1-phenyloctane. This PPE, according to 1H-NMR analysis on the end-groups, possesses an average contour length 20.3 nm long. Ut=1.4 V , average It= 0.55 nA. Besides the molecularly resolved areas there are zones with high molecular dynamics where the molecules are not immobilized at surface.

Histograms of the distribution of lengths of physisorbed rods. The PPEs posses an average contour length according to 1H-NMR analysis on the end groups, a polydispersity determined by GPC using poly(para-phenylene)s for calibration and a number of physisorbed rods as measured: (a) 7.95 nm / 1.92 / 593; (b) 11.2 nm / 1.84 / 253. The mole fraction of the Schulz-Zimm distribution is plotted in solid lines and in (a) Monte Carlo simulation (dashed and dotted lines). Dashed line: ratio of the potential of the interactions molecule-substrate and molecule-molecule = 2; dotted line: interaction molecule-molecule is reduced to zero. The normalization of the curves have been carried out on the tail of the distribution. Simulations have been performed using a repulsive hard core potential and London attractive forces. The model system was designed forming linear rods by overlapping several spheres linearly. The procedure was computed for a distribution of rods according to the Schulz-Zimm distribution. Calculations have been executed varying systematically the ratio between the potential of the interactions molecules-substrate and molecules-molecules in a range that spans from 2 to infinite.

Molecular Dynamics simulation of the evolution of the end-to-end distance of a single PPE in vacuum as a function of the increasing polymer length (expressed either in # of repeat units or contour length). The simulations have been computed at room temperature in vacuum applying the pcff force field (Molecular modelling package DISCOVER VERSION 4.0.0, Biosym Technologies Inc., San Diego, CA.) for a time scale of 100 ps for polymer up to 28 r.u., 150 ps for the molecule made of 35 and 42 r.u., and 250 ps for the polymer composed of 49 and 56 r.u.; these turned out to be enough for achieving a constant regime of energy fluctuations. The error bars represents the standard deviation of the end-to-end distance in the final 100 ps calculated.

Transmission Electron Microscope (TEM) micrograph (courtesy of Dr. C. Böttcher) of commercial Si tips (Digital Instruments). Tips displayed in a) and b) present a different shape. The average of the terminal tip radius of several tips has been evaluated as R = (13±7) nm.

Geometrical model for the broadening of the image of a ribbon due to the tip radius. R is the terminal tip radius, h is the mean height of the needles. = = (8±3) nm. The effective broadening results as 2=(16±6) nm.

SFM Tapping Mode height images representing the evolution of the morphology of the PPE on mica as a function of the polymer length. Average contour length of the molecule according to 1H-NMR results: a) 7.9 nm; b) 16.4; c) 20.3 nm. Height range of images: a) 20 nm; b) 30 nm; c) 20 nm. In case a) a mixture of THF and phenyloctane was used as a solvent while in b and c) pure THF was used.

Schematic representation of molecular ribbons adsorbed on the mica surface. a) Ribbons are made of several rods packed parallel to each other. b) Each rod is typically made of two PPE molecules packed with the hexyl lateral chains perpendicular to the basal plane of the substrate.

A) Tapping Mode SFM height image of an on average 11.2 nm long PPE on glass. Singular nanoribbon assembled on the grainy glass surface. B) TEM micrograph showing webs of ribbons on a carbon copper mesh support made by casting an on average 16.4 nm long PPE from a 0.1 g/l solution in THF.

Histograms of the distribution of ribbon widths for PPE from profiles on TEM micrographs. Contour length of the molecule according to 1H-NMR results and number of ribbons measured: (A) 7.9 nm / 164; (B) 16.4 nm / 167. Mole fraction (solid line) and weight fraction (dashed line) of the theoretical Schulz-Zimm distribution are in fairly good agreement with the experimental data.

Schematic representation of the molecular packing in the nanoribbon: a) Top view: a ribbon is composed by parallel, fully extended backbones, stacked perpendicular to the main ribbon axis; b) Side view: the lateral alkyl chains are disordered between adjacent backbones in a bilayer/trilayer aggregate standing on the substrate.

Valence Effective Hamiltonian (VEH) of the density of valence states (DOVS) simulated spectra and UPS HeII spectra of a spin coated film of pristine PPE trimer. The spectra are plotted relative to the Fermi level.

Valence Effective Hamiltonian (VEH) of the density of valence states (DOVS) simulated spectra of a doubly charged system and UPS HeII spectra of spin coated film of a doped PPE trimer (1.38 Na at/mon). The spectra are plotted relative to the Fermi level.

Adsorption of sulphur free PPE. a) Electrochemical adsorption induced by the applied voltage between the two electrodes; b) the self-assembly is likely to be governed by the chemisorption of sulphurs on Au nanoelectrodes.

Topographical Tapping Mode SFM image of a HBC-C12 surface produced by slow solution casting of one layer in controlled environment (route C in paragraph 5.3.3). h=15 nm. The white arrows indicate the preferential directions along which the layers grow anisotropic. The angle between them is 60° in perfect agreement with the three-fold symmetry of the HOPG substrate.